This invention relates to the cryogenic refrigeration of goods, particularly but not exclusively to the cryogenic refrigeration of goods in transit, and to an apparatus for refrigerating goods in a transportable container.
Known cryogenic refrigeration systems, for transporting perishable foodstuffs and the like, comprise heavily insulated liquid storage tanks which are capable of holding liquid cryogen at several atmospheres pressure. A distribution system is provided which utilises the pressure within the storage tank to generate a spray of cryogen within a container to reduce the temperature within the container, and that of any perishable goods therein. These systems are common in vehicular refrigerated trailers, or reefers, either instead of a or in combination with a mechanical refrigeration unit.
Such conventional cryogenic refrigeration systems are complex and expensive to maintain and operate properly, and there is a need for a more efficient way of utilising a cryogenic liquid for refrigeration of goods in transit without loss of refrigeration performance.
Accordingly, the present invention provides an apparatus for refrigerating goods in a container having means for storing a cryogenic fluid and means for discharging the fluid into the container in order to reduce the temperature therein, characterised in that the storing means is adapted to operate at atmospheric pressures but not at pressures substantially greater than atmospheric.
Such an arrangement, which can be embodied in a relatively simple and inexpensive liquid storage tank formed substantially of a foamed polymeric material, such as polystyrene, is more easily and safely operated than conventional cryogenic systems. Because the storage tank need only be capable of containing a little over atmospheric pressure (which may be assisted by the provision of an appropriately set pressure relief valve), the amount of cryogen it contains can be quickly gauged, with a suitable weight sensor, for example, and the problem of assessing how much of the cryogen is in the liquid state and how much in the gaseous state encountered in conventional systems is immaterial. So, operators of apparatus in accordance with the invention need less expensive training.
The delivery of the cryogenic fluid from the storage tank to the thermally insulated container for discharging onto the refrigerated goods may conveniently be effected by gravity. In a typical lorry or truck mounted system this can be achieved by mounting the storage means on top of the driver""s cab. Additionally or alternatively a pump may be provided to deliver the cryogenic fluid, however this may be disadvantageous as such a pump would require a separate power supply. Because the cryogenic fluid pressures in both the storage tank and the delivery lines from the storage tank to the discharge means are substantially atmospheric, these lines can be made inexpensively of plastics material, or of much thinner insulated metal than those of conventional, high pressure systems.
Discharge of the cryogenic fluid into the container may be by way of thermostatically controlled solenoid valves, as is known in the art.
A further problem with conventional cryogenic refrigeration transport systems arises where a simple vehicle is used to make a number of separate deliveries, each of a relatively small amount of refrigerated goods. With conventional xe2x80x9cmulti-dropxe2x80x9d systems there is frequent opening and closing of the insulated container to remove goods, and each time this occurs the refrigeration system is required to bring down the ambient temperature within the entire container, which is clearly inefficient. Therefore, in a further aspect this invention provides an apparatus for refrigerating goods in a transportable container comprising means for storing a cryogenic fluid and means for discharging the fluid into the container in order to reduce the temperature therein, wherein the container is subdivided into a plurality of discrete compartments, a plurality of discharge means being provided each being adapted to discharge cryogenic fluid into a single compartment.
With such an arrangement, when delivering a small load only one or two compartments need be opened and, after emptying of their load, either returned to their proper refrigerated temperature, or allowed to remain at ambient temperature without affecting the other compartments. This aspect of the invention may suitably be used in combination with the first invention described above.
Compartmentalisation of the container may be by simple, thermally-insulated partitions, however provision must be made for external access to each compartment without requiring any other compartment to be opened. This can be achieved by way of a standard sized thermally insulated container, or xe2x80x9cbox bodyxe2x80x9d, with a plurality of small doors each opening into a separate compartment. The provision of a plurality of compartments is also useful as they preferably have separate thermostatic control means for discharging cryogen for refrigeration; preferably these control means may be set to operate at different temperatures, meaning that different temperature regimes may be maintained in the different compartments, allowing goods which need to be refrigerated and delivered at different temperatures to be carried and delivered by a single containerised vehicle.
Each compartment in such an apparatus may be partitioned into upper and lower chambers. This is useful not only for further subdividing the space within a container but also enables different temperatures to be maintained very simply in associated chambers. Also, the internal partitioning may be movable, so as to enable the container to be configured into chambers of a range of different volumes.
Cryogenic fluid discharge means may be provided in the lower chamber of each compartment, with associated lower and upper chambers being in fluid communication for gas to flow therebetween. If, with such an arrangement, means are provided to discharge cryogenic fluid into the lower chamber, then this will rapidly chill. As cryogenic liquid in the lower chamber evaporates, low temperature gas (which is usually denser than air) will gradually rise to fill first the lower chamber and then, via suitable vents, to fill the upper chamber. Thus the lower chamber will be refrigerated rapidly whilst the upper chamber will cool more slowly. Providing appropriate vents to atmosphere from each upper and/or lower chamber, and actuating these in an appropriate manner, will allow different temperatures to be maintained in the upper and lower chambers as desired.
Another problem with vehicle refrigeration systems is that they require a means of maintaining refrigeration when the trailer-mounted container is disconnected from the vehicle tractor unit or when the vehicle""s engine is switched off. In the past, this has required a relatively costly and inefficient battery unit to be mounted to the trailer, or the provision of a complicated pneumatic control system.
Accordingly, in a third aspect, which may advantageously be combined with the first and second aspects of the invention set out above, the invention may provide an apparatus for refrigerating goods in a transportable thermally-insulated container including means for discharging cryogenic fluid into the container to refrigerate the container and goods therein, and elements having eutectic material configured and dispensed so as to act as a thermal buffer for the container, or for some or all of the separate chambers thereof.
Preferably, the roof and possibly the sides of the container, or of each compartment and/or each chamber, are lined with eutectic plates. These are usually two flat sheets of metal bonded together to form a relatively thin, flat tank which is filled with a eutectic material adapted to absorb and release heat at a constant temperature, preferably below 0xc2x0 C. (water, for example, has a eutectic temperature of 0xc2x0 C., at which it melts/solidifies whilst absorbing/releasing heat). Eutectic elements may be used so as to maintain different temperatures in different chambers, so as to allow a variety of different goods to be maintained at different temperatures.
In use, the cryogenic refrigeration system will maintain the refrigerated temperature of the goods within the container; some of the xe2x80x9ccoldxe2x80x9d will also leak into the eutectic plates where it will be stored. Eventually the eutectic plates will have xe2x80x98storedxe2x80x99 as much cold as possible, ie all the material will have been converted from one state (usually liquid) to another (usually solid), and they will then perform an insulation function. When the container is disconnected or the vehicle engine switched off, the cryogenic refrigeration system will no longer function but the eutectic plates will yield cold to keep the container at the derived temperature. Depending on the number of plates and the volume of eutectic material, the duration of this xe2x80x9ceutectic refrigerationxe2x80x9d can be extended or reduced as required.
The invention will now be described by way of example and with reference to the accompanying drawings.